Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997.
A particular type of fluoropolymers comprises copolymers of a fluorinated olefin and ethylene and/or propylene. Fluoropolymers of fluorinated olefins and ethylene or propylene have been described in for example U.S. Pat. No. 4,277,586, GB 1,291,936, U.S. Pat. Nos. 3,825,510, 4,463,144 and 4,148,982. According to the teachings of these publications, copolymers of a fluorinated olefin such as for example tetrafluoroethylene (TFE) and ethylene (E) or propylene (P) can be prepared through various polymerization methods including aqueous emulsion polymerization. Generally, the aqueous emulsion polymerization is conducted in the presence of a fluorinated surfactant as is disclosed in for example U.S. Pat. Nos. 4,277,586 and 4,463,144.
Although the aqueous emulsion polymerization is generally conducted in the presence of a fluorinated surfactant, aqueous emulsion polymerization processes have recently also been developed that avoid the use of a fluorinated surfactant. Such polymerization processes offer the advantage that the resulting product is free of the fluorinated surfactant, which may be desired from an environmental point of view. So called emulsifier free aqueous emulsion polymerizations have been disclosed in for example U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381.
It has however been found that the aqueous emulsion copolymerization of a fluorinated olefin with ethylene and/or propylene does not proceed well or does not proceed at all when no fluorinated surfactant is added to the polymerization. Accordingly, it would be desirable to find an improved process in which copolymers of a fluorinated olefin and a hydrocarbon olefin selected from ethylene, propylene or mixtures thereof can be readily prepared in an emulsifier free aqueous emulsion polymerization.